Conventionally, an engine is known which comprises a variable valve timing (VVT) mechanism in which an open/close timing of a suction valve is set to be variable. Such an engine is used as a running power source of an automobile.
In the variable valve timing mechanism, an engine power and a gas exhaust capability are optimized according to the number of rotations of the engine, by setting a rotational phase angle of a cam shaft to which a cam for opening or closing the suction valve is fixed to be variable such as advancing (advance angle) or delaying (lag angle) the rotational phase angle of the cam shaft with respect to the rotational phase angle of a crankshaft. In such a variable valve timing mechanism, when hydraulic control is used for changing the phase angle of the cam shaft, in many cases, lubricant oil which is used for lubricating a piston or power transmission system in the engine is also used as the operation oil for the hydraulic control. In this case, a foreign object contained in the operation oil, such as metal powder, may be clogged in the portion which is movable by the hydraulic control, and the variable valve timing mechanism may not be normally operated.
For example, JP 2001-254637 A discloses a valve characteristic control device of an internal combustion engine in which, when it is judged that there is abnormality in the variable valve timing mechanism, the variable valve timing mechanism is forcefully driven, for a foreign object removal process.
JP 2005-220759 A discloses a lift amount variable control device of an internal combustion engine in which, when the lift amount of a suction valve is fixed at a low lift side due to failure of a variable valve mechanism, a target number of idle rotations is corrected and increased, to avoid occurrence of stalling of the engine.
When the engine having the variable valve timing mechanism as described above is equipped in a hybrid vehicle which has a motor as a separate running power source which is driven by electrical power from a battery, even during running of the vehicle, an intermittent operation control is executed in which the operation of the engine is stopped during EV running in which the vehicle is run with the motor power. If the operation of the engine is stopped in the middle of the foreign object removal process as disclosed in JP 2001-254637 A when abnormality occurs in the variable valve timing mechanism in such a hybrid vehicle, the foreign object removal process cannot be properly executed. Because of this, problems are expected in which a user who is notified by the vehicle that failure has occurred in the variable valve timing mechanism may complain, and cost for solving such occurrence of the failure may be increased.
In addition, because the above-described foreign object removal process is control involving degradation of exhaust, in particular countries, laws require that the failure judgment of the variable valve timing mechanism be established by completing the foreign object removal process within a predetermined time period (for example, 10 seconds). In such a case, if the engine is stopped in the middle of the foreign object removal process as described above, the possibility of judgment of failure of the variable valve timing mechanism is high even through the abnormality can be solved by executing, to the maximum possible extent within the predetermined time period, the foreign object removal process.
Moreover, in JP 2005-220759 A, as described above, the target number of idling rotations is corrected and increased when the variable valve mechanism is fixed, to avoid stalling of the engine. However, although such a configuration has an advantage of inhibiting passive stopping of the engine operation during idling, this configuration cannot be applied to a vehicle which executes the intermittent operation control to actively stop the engine operation, in cases not limited to the time of idling, in order to improve the fuel consumption.